When bonding reinforcing fibers together by a resin matrix to create a fiber reinforced polymer composite, the presence of functional groups on the fiber's surface is very critical. In addition, the bond has to be durable when subjected to environmental and/or hostile conditions. Bond strength, i.e., the force per unit of interfacial area required to separate the (cured) resin from the fiber that is in contact with the cured resin, is a measure of adhesion. Maximum adhesion is obtained when a cohesive failure of either the resin or the fiber or both, as opposed to an adhesive failure between the fiber and the resin, is mainly observed.
To make a strong bond, firstly oxygen functional groups are beneficially introduced on the pristine fiber's surface; secondly an adhesion promoter may be selected such that one end of the adhesion promoter is capable of covalently bonding to the oxygen functional groups on the fiber's surface while another end of the adhesion promoter is capable of promoting or participating in chemical interactions with functional groups in the resin. Essentially, the adhesion promoter acts as a bridge connecting the fiber to the bulk resin during curing. A surface treatment such as plasma, UV, corona discharge, or wet electro-chemical treatment is often used to introduce oxygen functional groups onto the fiber's surface.
Ultimately, to achieve the strong bond, there certainly cannot be voids at the interface between the fiber and the resin, i.e., there is sufficient molecular contact between them upon curing. Often, this interface is considered as a volumetric region or an “interphase”. The interphase can extend from the fiber's surface a few nanometers up to several micrometers, depending on the chemical composition on the sized fiber's surface, chemical interactions between the fiber and the bulk resin, and the migration of other chemical moieties to the interface during curing. The interphase, therefore, has a very unique composition, and its properties are far different from those of the fiber's surface and the bulk resin. Moreover, the existence of high stress concentrations in the interphase due to the modulus mismatch between the fiber and the resin often makes the composite vulnerable to crack initiation. Such high stress concentrations could be intensified by chemical embrittlement of the resin induced by the fiber, and local residual stress due to the thermal expansion coefficient difference such that when a load is applied, a catastrophic failure of the composite can be observed.
Conventionally, inadequate adhesion might allow crack energy to be dissipated along the fiber/matrix interface, but at the great expense of stress transfer capability from the adhesive through the interphase to the fibers. Strong adhesion, on the other hand, often results in an increase in interfacial matrix embrittlement, allowing cracks to initiate in these regions and propagate into the resin-rich areas. In addition, crack energy at a fiber's broken end cannot be relieved along the fiber/matrix interface, and therefore, diverted into neighboring fibers by essentially breaking them. For these reasons, current state-of-the-art fiber composite systems are designed to allow an optimal adhesion level.
In some cases, especially involving carbon fibers, weak to intermediate adhesion levels are desired so that there is a balance between adhesion related properties such as tensile strength and compressive properties such as compression strength and open-hole compression (OHC) strength. Typically a higher resin modulus mainly gives rise to higher compressive strengths. However, up to a certain (flexural) resin modulus between 4-5 GPa, these strengths are leveled out and will not increase further. The reason could be due to a weak interphase that is not suitable to prevent fibers from premature failures from buckling. A high resin modulus, on the other hand, could cause the polymer to be brittle and therefore could result in low tensile-related properties as well as low fracture toughness.
Recently, composite materials have been utilized successfully in commercial aircrafts due to their high specific strength and stiffness over metal alloys, as seen in Boeing 787 and Airbus 380 and 350 aircrafts. More specifically, carbon fiber composite materials allow designs of thin and high aspect ratio wings that could not be achieved with metal alloys, resulting in better aerodynamic efficiency by drag reductions. Such designs are anticipated to require high torsional and bending stiffness of the composite materials. Therefore, there are needs to overcome the above barriers in compressive strengths as well as tensile-related strengths and fracture toughness.
WO2012116261A1 (Nguyen et al., Toray Industries Inc., 2012) attempted to create an interphase by a self-assembled process in which an interfacial material is incorporated into a resin and by utilizing surface chemistry of a reinforcing fiber to concentrate the interfacial material in the vicinity of the fiber. This process has been shown to be robust, forming a reinforced interphase that in turn could simultaneously improve tensile strength and fracture toughness of the composite by utilizing a rubbery interfacial material. However, since this soft interphase was created, it would not have been able to effectively prevent fibers from buckling under a compression load. In addition, though the interphase's modulus could not be measured directly, such a soft interphase while having high fracture toughness could have low modulus, and thus could reduce load transfer capability of the polymer to the reinforcing fiber, especially when the fiber is a high modulus carbon fiber. As a result, there is a need to create a hard interphase, without penalizing too much of its fracture toughness, that could simultaneously provide good adhesion hence high adhesion-related properties and improve compressive properties.